Just look at how Lib Dem supporters are splitting
The forced choice question that appears above is one that’s been included in the Telegraph’s monthly YouGov poll for several years now and I think is probably a good guide to potential tactical voting.
For this asks simply: “If you had to choose which would you prefer to see after the next election a Conservative government led by David Cameron or a Labour government led by Gordon Brown?”. For me the key responses are from Lib Dem supporters because this gives a sense of potential tactical voting in seats where it’s mainly a straight CON-LAB contest.
As can be seen there’s now a significant CON/CAM lead. In February 2009 the split was even at 41-41 with the rest saying they didn’t know.
We haven’t got much detail of party breakdowns when similar questions were put ahead of the 1997. 2001 and 2005 general election but my guess is that the Lib Dems would have overwhelmingly favoured Labour. Even in February 2006, after Cameron had become Tory leader, BROWN/LAB was leading by a ratio of 5:2.
Now the uniform national swing, the main concept behind the simple seat calculators, takes what happened last time and applies the national swing on an additive basis.
So any tactical or antiTory voting that happened in May 2005 is frozen in and the model uses that to apply the changes. But if there are underlying trends like the one above then we might not be getting the full picture.